SER or NM ?

T

TwistLock

Member
Location
California
Occupation
Electrician
I typically see SER with bare ground. In photo below the ground is insulated. Since this place was built in the ’70’s I’m just curious if there was NM available in #2 AL and or a distinction between SER back then - and if it even matters for the 83% rule, formerly 310.15(b), for a feeder from a meter-main.
I.e. if this were #2 AL NM (not listed as SER) would I still be allowed to protect it with a 100a breaker at meter-main or is it just a stepchild at 60C ? *(can't see any markings on jacket with what is visible at either end).

SER or NM rated.jpg
 
"...What size is the ground? Looks undersized for SER which usually has a larger ground than NM "

It’s a job someone walked on & I’m thinking of taking it over so I took some quick photos and did measure the neutral while there (pic below), but didn’t bother with the ground. You’re right though, it’s hard to tell if ground is #8 or #6. Maybe they used a really thin jacket, compared to the thick jacket on CCC’s, back then since it was the ground(?)


" It looks like a larger version of the same cable type which is right next to it on the right side in the photo. That jackets says NM 600V. "

Yes, that caught my eye too, they're buddies.

Neutral awg.jpg
 
SER ground is always equal or 2 AWG sizes smaller than the main conductors since it is intended to be used as neutral return in a service. So what you have there has to be NM cable. Also SER has fiberglass tape and the picture appears to be paper in the cable which would make it NM. SE cables have no paper since they are intended to be outdoors.
 
TwistLock said:
for the 83% rule, formerly 310.15(b), for a feeder from a meter-main.
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Whats the calculated load for that panel?
Since you cant use the 220.82 optional calc you can use 220.83.
Are there any other breakers in the meter main? Is it being replaced also?
 
I had done some work in a mobile/manufactured home not too long ago that originally had aluminum NM cable, which most of which I ended up removing. All the 12-2 and 10-2 had bare solid aluminum EGC but larger cables had reduced size stranded aluminum EGC with green insulation on them.

Must have been the trend to use green on those EGC's larger than 10 AWG when it was aluminum conductors?
 
There's some restrictions on this job. I will be replacing meter main(s) but can't do anything about lateral service from pad x-former to meter main, nor can I replace the #2 NM AL panel feeder (first photo) due to difficulty and cost.
Tortuga pointed out 220.83 (thank you), so I'll do the math. But my question is, is there anything allowing me to treat this existing NM feeder as anything other than 60C / 75amp ?
 
In the 1968 NEC, under table 310-14, there is an asterisk that says 2AL THW can be used for a 100 amp service. (75C)

If you take the 83% rule in the modern 60C, you get 90 amps.

Any reason you couldn't use 90A on this one?
 
Others looking at project have come up with 100a, but I cant understand how yet unless they mistook cable for SER.
To my, possibly limited, understanding 310.12(A) doesn't negate NM 60C. 220.83 might help wring out a few amps but when I see there's been a kitchen remodel & or one or two circuits added etc. that's a wash.
90a (74.7 @ 83%) is all I can come up with too.
 
2020 NEC 310.12(A) and (B) say at the end "If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied." And Table 310.12 says #2 Al is allowed for a 100A residential service or main feeder, without regard to the insulation rating.

This is a loophole, as the intention was that Table 310.12 applies to the case of minimum 75C rated conductors. That makes the sizes listed correspond to the 83% factor as specified in the rest of 310.12(A)/(B). The 2026 NEC First Draft incorporates a change to close this loophole.

Cheers, Wayne
 
Man that clears the cobwebs - thanks. I would have never presumed that 'table' 310.12 included NM. (I hope they do close this loophole, that's some b.s.)
 
wwhitney said:
This is a loophole, as the intention was that Table 310.12 applies to the case of minimum 75C rated conductors.
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The normal 310.16 table is "Not More Than Three Current-Carrying (CC) Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)".
If you look back at the 1956 when they added it as a note to table 1a in chapter 10 it was basically an ampacity adjustment for Two CC Conductors, they did not want to extend it beyond residential services, though you could use engineering supervision to probably come up with the same adjustment.
wwhitney said:
The 2026 NEC First Draft incorporates a change to close this loophole.
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Thats too bad, the CMP is making a mistake there, the only problem with it was when they added open wye 208 services, which is no longer 2 CC in a raceway or cable.
 
tortuga said:
Thats too bad, the CMP is making a mistake there, the only problem with it was when they added open wye 208 services, which is no longer 2 CC in a raceway or cable.
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Wait, why is it a mistake? 75A / 0.83 < 100A. So even with a 1/0.83 ampacity adjustment factor for only 2 CCCs, 60C #2 Al shouldn't be allowed to be rated at 100A.

Now, you could argue that the 60C limit on NM cable is itself a mistake, which I might be inclined to agree with. But that's a different matter.

Cheers, Wayne
 
wwhitney said:
Wait, why is it a mistake? 75A / 0.83 < 100A. So even with a 1/0.83 ampacity adjustment factor for only 2 CCCs, 60C #2 Al shouldn't be allowed to be rated at 100A.

Now, you could argue that the 60C limit on NM cable is itself a mistake, which I might be inclined to agree with. But that's a different matter.

Cheers, Wayne
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When they added it to the code in 1956 they did not use the 83% factor, it was based on testing 2 CC's and back then many 60C insulation types were very common such as TW and R. So now in 2024 that is what 68 years #2 AL has been allowed to be protected by a 100A OCPD? and we dont seem to have daily posts about houses with #2 AL burning down. Then when they allowed open wye it seems they forgot the history of the ampacity adjustment.
 
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